The present invention relates to an apparatus for controlling the hydraulic drive of a construction machine and, more particularly to a hydraulic drive controlling apparatus with a load sensing system, which controls the capacity or displacement of a hydraulic pump or pumps in a construction machine, e.g. a hydraulic excavator and the like, in such a manner that the discharge or delivery pressure of the hydraulic pumps becomes higher by a fixed value than the maximum one of load pressures of plural actuators.
A construction machine, for instance a hydraulic excavator, is equipped with one or a plurality of hydraulic pumps. Actuators of the machine, such as a boom cylinder, an arm cylinder, bucket cylinders, a swing motor, left and right traveling motors, etc., are driven with the hydraulic fluid delivered from these hydraulic pumps. A directional control valve is provided between each actuator and the hydraulic pumps. Operator's manipulation of a control lever for each actuator at his discretion, which control lever is provided in an operator's cabin of the hydraulic excavator, operates the corresponding directional control valve responsively. The thus operated directional control valve controls the flow of the hydraulic fluid from the hydraulic pumps to the actuator, and hence the movement thereof is controlled to perform an expected operation of the hydraulic excavator.
Various systems have been proposed for the drive control of such actuators. Typical one of the thus proposed is the load sensing system shown in JP-A-60-11706 which corresponds to U.S. Pat. No. 4,617,854. This load sensing system is adapted to control the displacement of the hydraulic pumps during the drive control of the actuators so that the delivery pressure of the hydraulic pumps is kept higher by a fixed value than the maximum one of load pressures of the actuators. By virtue of this control, the actuators are driven with the minimum delivery of the hydraulic pumps to enable an economical operation.
As described above, the load sensing system is extremely superior as a hydraulic drive controlling apparatus for a construction machine. This system, however, can not control the driving pressure of each actuator in response to the corresponding control lever. Accordingly, in a certain case, for instance, when the load sensing system is employed in the drive control of an actuator for moving a member of large inertia, the following inconvenience or disadvantage arises.
That is, when it is desired to slowly accelerate the swing motor for driving a swing body of a hydraulic excavator, which body is of large inertia, even a slight amount of manipulation of the corresponding control lever causes the delivery pressure of the hydraulic pumps to abruptly increase until the load pressure of the swinging motor reaches a relief pressure for the swinging. This is because the control of the pump displacement is done so as to increase the delivery pressure of the hydraulic pumps by a fixed value as compared with the load pressure of the swing motor. As a result, the high relief pressure is applied to the swing motor, and the same is suddenly accelerated, despite the manipulation for a slow acceleration, to make a different motion from operator's intention. Such motion is quite dangerous. In addition, a large quantity of the hydraulic fluid is relieved, resulting in a large loss of power.
Similar phenomena arise in the case of the other actuators. One example given here among many operations of the hydraulic excavator is the operation of softly pressing its bucket against dug portions of the ground to level the same. Also in this operation, as the driving pressure can not be adjusted, the working member is brought to press rather hard against the ground, making it difficult to perform the desired working while causing a great loss in power.
It is an object of the present invention to provide a hydraulic drive controlling apparatus for a construction machine, which is capable of adjusting the driving pressures of actuators of the machine, while making efficient use of the load sensing system's merits, to enable the actuators to do expected motions.